The invention relates, in general, to the control of large rotating shaft machinery and, in particular, relates to a testable, redundant trip circuit for a turbomachine.
In a turbomachine, electronic speed control is provided on two levels; namely, speed control based on normal operating conditions wherein control valves are positioned in accordance with a comparison between a desired speed and an actual speed and the range of control is normally from 0-100 percent of rated speed. In response to a valve position change command, the turbine valve may be adjusted to a more open or more closed position. On the other hand, a second speed control relates to overspeed protection which pertains at speed in excess of rated speed and which, when reached, will cause a turbomachine to trip based on the rapid shutting down of motive fluid to the turbomachine. Overspeed protection is usually provided in the form of a solenoid valve between the source of motive fluid and the turbine which is normally full open during normal operating conditions and which will fully close in response to a signal from an overspeed trip circuit. It is this latter type of speed control to which this invention is directed.
Some turbomachines are used in support of other operations such as boiler feed pump turbines, industrial drive turbines and plant process turbines. The underlying common concern regarding these types of support turbines is that a shutdown of the turbine due to an overspeed condition or turbine control failure may cause a chain of shutdown events which can be very costly to the turbine owner.
Reliability can be expressed in two ways; namely, the ability to trip the turbomachine when an overspeed condition arises; and, the ability not to trip the turbomachine unless an overspeed condition arises.